Plastic materials have been widely used in various optical applications, particularly in manufacturing spectacle lenses, because of their light weight, toughness and easiness of dyeing. Optical products, particularly spectacle lenses are required to have, in addition to a low specific gravity, a high clearness, a low yellowness, optical properties such as a high refractive index and a large Abbe's number and physical properties such as high heat resistance and large mechanical strength. A large refractive index can decrease thickness of a lens. A large Abbe's number is important to avoid chromatic aberration of a lens. A high heat resistance and a large mechanical strength are important to facilitate fabrication and also for safety.
The inventors found a novel episulfide compound capable of providing a thin optical material which shows little chromatic aberration and has a refractive index of 1.7 or more and Abbe's number of 35 or more, as disclosed in Japanese Patent Application Laid-Open Nos. 9-71580, 9-110979, and 9-255781. The proposed episulfide compound, however, tends to increase its viscosity by spontaneous polymerization during the storage or injection operation because of a high reactivity of the episulfide group. The viscosity increase by the spontaneous polymerization becomes considerable, particularly, after the addition of a polymerization catalyst, thereby making it difficult to perform a long-term molding process. The episulfide compound is cured generally by ion polymerization. The polymerization rate of ion polymerization is difficult to control. Therefore, particularly in the production of a resin for optical material, optical distortion, striae, coloring, separation from mold during polymerization, etc. are likely to occur to reduce the yield of product.